drm/msm: enable support for custom power ioctl

This change adds support of msm_power_ctrl ioctl from msm display
driver. This ioctl is required to enable/disable display power
before/after accessing display core registers by clients.
TZ asynchronously accesses display domain registers without display
driver knowledge.This ioctl provides any such clients capability
to add/remove vote on display core clocks and power rail.

Change-Id: Ibf564ee3ad44884da55bf8a1e62f9d409ecf947b
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>